Low Net: Gross Reservoirs of the Transgressive Shelf Systems in the Upper Mancos Shale, El Vado Sandstone, San Juan Basin, New Mexico

Abstract

The El Vado (EV) Sandstone Member (upper Coniacian) of the Mancos Shale has been considered both a conventional and unconventional hydrocarbon resource target (Ridgley, 2001) in the San Juan Basin (SJB) of northwestern New Mexico. Sandstones of the EV were deposited on the marine intracontinental shelf along the west margin of the Cretaceous Seaway. The EV is part of a transgressive-regressive wedge of rock that overlies a large, older transgressive wedge that contains the Tocito Sandstone. The EV is examined in outcrops in the northeast and the southeast parts of the SJB. In northern outcrops, the EV consists of five parasequences of interbedded sands and shales of similar thickness (∼4 m each) separated by marine-shale units that become progressively sandier upward. To the south, the EV Sandstone transitions from a shelf-deposited into a lower-shoreface sandstone. Well logs and core are used to map the subsurface distribution of the EV Sandstone in the SJB. It is reflected in logs as a thick (∼36.6 m; 120 ft) interval of low gamma ray and high resistivity, associated with highly laminated sand to silt interbedded with shales. A total of 13 cross sections are correlated and used to construct total-interval isopach and parasequence-scale isopach maps for the EV. Regional net-sand and net:gross maps, determined from 104 digitized well logs, highlight the regional distribution of sand in the formation. Thicks are oriented north to south and show compensated deposition suggesting that individual parasequences are not in communication across the basin, but opportunity exists to penetrate stacked parasequences with single vertical wells.